One of the prior processes for preparing cyclic aryloxy phosphazenes is a process of Yokoyama (Journal of the Chemical Society of Japan, Vol. 80, No. 10, p. 118 (1959)) in which phosphorus pentachloride is reacted with ammonium chloride in monochlorobenzene to produce a mixture of cyclic chlorophosphazenes represented by formula (3)
wherein m is a integer of 3 to 10, with linear chlorophosphazenes represented by formula (4)
wherein n is an integer of 1 to 20; and the mixture of cyclic and linear chlorophosphazenes is recrystallized in a petroleum ether, etc. to produce chlorophosphazenes represented by formula (3)
wherein m is an integer of 3 to 10; and the chlorophosphazenes are reacted with phenols to produce cyclic aryloxyphophazenes. Concretely, Yokoyama (Journal of the Chemical Society of Japan, Vol. 81, No. 3, p. 481 (1960)) reports on a condensation reaction of triphosphonitrile chloride with alkaline metal phenolates in the presence of pyridine as catalyst.
In addition, Japanese Patent Laid-open No. Sho 58-219190 discloses polyhydroxyphenyl phosphonitrilate bearing phenolic hydroxy groups characterized by containing substantially no polycondensation products and no residual chlorine; and a process for preparing the same. This relates to polyhydroxyphenyl phosphonitrilate bearing phenolic hydroxy groups and a process for preparing the same characterized by comprising reacting a phosphonitrile halide with a sodium or potassium salt of a monomethoxy phenol obtained by protecting one hydroxy group of a dihydric phenol with methyl group to produce polyhydroxyphenyl phosphonitrilate, and then reacting the phosphonitrilate with a pyridine hydrohalogenic acid to convert the methoxy moiety into a hydroxy group.
The present inventors disclose in Japanese Patent Laid-open No. 2001-2691 a process for preparing a cyclic phosphazene comprising reacting a cyclic halogenated phosphazene with an alkaline metal phenolate in a solvent containing nitrogen-containing linear and/or cyclic organic compound.
In the prior processes for preparing cyclic aryloxyphosphazenes, isolated cyclic chlorophosphazene is used as a raw material. When cyclic chlorophosphazenes are obtained in an industrially large amount from a mixture of cyclic and linear chlorophosphazenes, a large quantity of halogen gas may be generated and pollute remarkably the environment, and results in very low workability. Even though isolated and purified cyclic chlorophosphazenes are reacted with a phenol, the intended cyclic phosphazene is not prepared because linear phosphazenes are given by ring-opening of cyclic phosphazenes during the reaction, unsubstituted phosphazenes having chlorine may remain, or linear phosphazenes are given by the hydrolysis of cyclic phosphazenes during its after-treatment.
Phenyl phosphonitrilates prepared by the process described in Journal of the Chemical Society of Japan, Vol. 81, No. 3, p. 481 (1960) are not solid but yellow viscous liquid as they contain linear phosphazenes prepared by ring-opening of cyclic phosphazenes, or unsubstituted phosphazenes having chlorine, and the phosphonitrilates cannot be purified.
A preparation process disclosed in Japanese Patent Laid-open No. 2001-2691 discloses phosphazenes containing lower content of halogen compared with the conventional processes, but the content of halogen is as much as about 100 ppm. In addition, as cyclic halogenated phosphazenes are used as a raw material, it is unavoidable to result in linear phosphazenes due to ring-opening of a part of the cyclic phosphazenes during the reaction.
In addition, in Japanese Patent Laid-open No. Sho 58-219190 described above, a sodium salt of monomethoxy phenol is used in an amount by a factor of 1.1 based on 1 of chlorine in phosphonitrile halide and the reaction is carried out. However, the reaction is not completed and thus unsubstituted organic halides remain. Consequently, although phosphazenes represented by the above-mentioned general formula (1) contain no halogen in the structural formula thereof, all substituents thereon cannot be necessarily substituted with aryloxy groups, so that halogens still remain on a part of the resulting compounds in the structural formula thereof. In particular, it is difficult to substitute the finally remaining one halogen on a compound, and it is not able to completely substitute even when the reaction is carried out for about 250 hours.
Consequently, although the phosphazenes represented by the above-mentioned general formula (1) are cyclic on the structural formula thereof, the cyclic structure on the above-mentioned general formula (1) cannot be necessarily maintained in the conventional preparation processes, therefore a part of the products becomes linear compounds.
Additives for resin include flame retardants, UV light absorbers, antioxidants, plasticizers, nucleating agents, etc. Needless to say, they have to satisfy several requirements, for example they do not deteriorate stability during fabrication nor pollute molds, further they affect no adverse influence on products, in particular they cause no breed-out that they exude from the surface of resins.
In particular, recently it has become problems that flame retardant compositions containing a resin and a flame retardant in which halogen-containing compounds that have been conventionally main substances for the flameproofing of wires or cables, etc. are used alone or an admixture with antimony compounds, such as antimony oxide generate halogen gas when the compositions are subjected to combustion or molding process, etc. It is further said that the generated gas often deteriorates electrical properties or transmission properties. Therefore, there has been a growing demand for a flame retardant resin composition that generates no halogen gas when it subjected to combustion or molding process, etc.
In order to fulfill these demands, for recent years metal hydrates or phosphorus flame retardants (phosphate esters, ammonium polyphosphates, phosphazenes, etc.) have been used as non-halogen flame retardants.
Metal hydrates, such as aluminum hydroxide or magnesium hydroxide have been regarded to improve a flame retardant effect because endothermic reaction due to dehydration and thermal decomposition of the metal hydrates at a combustion temperature of a resin occurs within a temperature range overlapping with a temperature at which a resin starts thermal decomposition or combustion. However, as the effect affording flame retardance by the metal hydrates alone is not very high, they have to be added in a large quantity, thereby resulting in defects that molded products are adversely affected on the mechanical properties thereof.
The conventional phosphorus flame retardants have excellent properties that act as plasticizers or antioxidants by adding to a resin in addition to inherent flame retardant effect. However, as triarylphosphates, such as triphenylphosphate, tricresylphosphate or cresyldiphenylphosphate have a low boiling temperature, they have defects that they vaporize during the molding process and thus pollute molds or they cause breed-out which they exude from the surface of resins. Condensed phosphate esters resolve the above-mentioned defects. However, when the catalysts used in the preparation of the phosphate esters remain therein, it is known that they result in a lowering of performance due to decomposition of not only the phosphate esters but also resins during the molding process and that the phosphate esters gel, thereby causing a remarkable lowering in productivity. Ammonium polyphosphates have a low heat-stability and thus impose restraints on processing conditions. In addition, as ammonium polyphosphates have a low content of phosphorus, they have also a defects that have to be added in a large quantity.
An object of the present invention is to provide cyclic phosphazenes which solve the defects in the prior technology as mentioned above, can be produced without repeated purification, have an extremely low content of halogen and have a low content of linear phosphazenes; novel processes for preparing the same; flame retardants containing the same as an active ingredient; and resin compositions, containing the cyclic phosphazenes and molded articles therefrom.